Rudy, a 60-something Englishman with a pierced ear and a bandana, bought his first vehicle in 1966 - a second-hand Barnstaple school bus with no suspension and dodgy brakes. It took him as far as Maidstone before the engine caught fire. The recovery mechanic enquired how far he intended to go: Rudy told him Lahore. The mechanic replied he'd be lucky to reach Southend.

In fact, the bus eventually limped into Istanbul before juddering to a halt. But having made it so far along the trail, there was no shortage of hippies, drop-outs and fellow travellers eager to hitch a ride, even if it meant pushing the bus to Pakistan.

The eastwards migration of Rudy, and thousands of long-haired western youths like him, is a cultural phenomenon that has never been properly accounted for. In 1967, the Indian government estimated that there were some 10,000 "youthful foreigners" floating around the country. That was before the Beatles arrived - by the end of the decade, the same number poured over the Pakistan border in a single week.

Few of these travellers knew precisely where they were heading - this was the original unpackaged tour, with no backpacker posts, Lonely Planet books or internet cafés for guidance. Instead, inspired by the work of Kerouac and Ginsberg and the sound of sitars on Beatles records, young explorers set out in search of cheap drugs and spiritual enlightenment, transforming the ancient highway of the Silk Road into a psychedelic caravan of badly maintained vehicles.

Today, such idealistic overland drifting is no longer possible. Iran closed its borders to travellers after the revolution in 1979. Afghanistan became a lawless battleground for the world's superpowers. Where the first wave of hippies might be welcomed into the homes of Kurdish tribesmen as honoured guests, Islamic terror groups now identify tourists as a legitimate target. Yet perhaps the biggest blow to the hippy ideal was the realisation that there really was no Shangri-La - or at least there wasn't until the Chinese authorities recently designated the anonymous Himalayan town of Zhongdian as an official tourist paradise.

None of this has deterred Canadian travel writer Rory MacLean from attempting to retrace the hash-and- hepatitis trail laid down by the original flower generation. It's the kind of expedition that brings to mind Tim Moore's barmy schemes for crossing Spain on a donkey, or touring Italy in a purple suit; and it's actually a little disappointing to discover that MacLean does not propose to undertake the trip in a decrepit VW Campervan with peace symbols painted on the side. He relies instead on local bus and rail services, with occasional recourse to short air hops, which smacks of cheating, though nobody would call an Afghan Airlines flight to Kabul a safe bet.

In fact, this scheduled flight never gets off the ground - the airline's sole 747 having been borrowed by a government minister for private business - so Maclean has to talk his way aboard a UN flight carrying a Danish MP which is forced to land at the US air base at Bagram. Bizarrely it is here, as a guest of the American military, that he undergoes his first truly psychedelic experience.

Sitting in the mess tent, he reveals the nature of his mission to the Danish MP, who replies: "The hippy trail? I wouldn't be here now if I hadn't hitched to India in 1967!" It soon emerges that the servicemen had recently enjoyed a 60s evening on the base, and before long everyone is grooving to "The Age of Aquarius" in a selection of long wigs and paisley waistcoats, while stealth bombers take off from the runway outside.

The incident encapsulates how deeply the counter-culture left its mark on modern society, though whether for good or ill remains a matter for debate. MacLean talks to a Turkish tour operator who states simply that "our prosperity started with the arrival of the hippies"; yet he also quotes Bruce Chatwin's belief that Afghanistan's descent into anarchy could be blamed on "pony-tailed, peace-loving hippies driving educated Afghans into the arms of the Marxists".

MacLean balances these arguments in a tone which falls somewhere between celebration and regret, eulogising the hippy trailblazers who established the path to enlightenment, yet in doing so ensured that there were no trails left to blaze. He salutes the enterprise of people prepared to travel thousands of miles without adequate suspension ("the secret for a successful trip was to get the passengers smoking chillum dope pipes before breakfast"); yet he observes how four decades of tourism have turned Tibet into a Himalayan theme park.

Ultimately, one is left with the sad ruminations of Rudy, the Barnstaple bus driver, forced out of business by the Iranian revolution: "All the guys stopped driving then," he says. "George on Budget Bus, Ugly Bob from Chattanooga who had the Volvo. It broke my heart having to stop." Ugly Bob with the Volvo, wherever you are - this one's for you.

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